Cone-beam computed tomography (CBCT) are widely employed and have many applications. In the past decade, they have become increasingly popular in the dental industry in many diagnostic and treatment procedures, such as dental implants, temporomandibular joint, orthodontics, impaction, orthognathic surgery, caries diagnosis, endodontics and the detection of periapical pathosis.
Dental cone beam computed tomography, also called cone beam volumetric tomography (CBVT), uses a single sweep of a conical x-ray beam around the patient's head to acquire a volume of anatomical data, e.g. a patient's jaws or part thereof. Similar to single or multi-slice CT scanner, the x-ray sensors or detectors in CBCT scanners are positioned opposite of the x-ray source to collect a map of x-ray absorption through the head of the patient. The central axis of the patient's anatomical volume of interest determines the locations of the x-ray source and sensor. The axis of rotation of source and sensor needs to correspond to the central axis of the volume of data.
Image data from CBCT is produced by reconstruction software, which creates a data set for a given volume that is called the field of view (FOV). Typically the quality of the image data is at its best towards the center of the FOV. Contemporary CBCT scanners are available with a variety of FOVs ranging from small, e.g. 4 cm diameter by 4 cm height, to large, e.g. 17 cm diameter by 23 cm height.
In standard usage the patient is aligned in the scanner, which may require the patient to stand, sit or lie to position their head between source and sensor. The position of the patient in the scanner is then adjusted so that the desired structures lie within the boundaries of a selected field of view. This is achieved by moving the patient's head in the FOV or moving the FOV over the patient's region of interest.
The FOV and the patient's region of interest are typically aligned with the aid of e.g. laser alignment beam projecting an illuminated line onto the patient's face. Where only laser beams are used to align the patient there are usually provided two or three intersecting laser “lines” which represent the visible projection of a 2d projection of a plane onto the patient's face, which typically run in X Y and Z alignment. Typically the central axis or the center of the volume is defined by the intersection of these lines, but alternative alignments may be sometimes required, depending upon the precise alignment and set-up of the system. The FOV and patient's position may be confirmed or improved with “scout” view exposures. The “scout” views are lateral or anterior-posterior plain x-ray projections on to the sensor that allow the operator to check that the region of interest actually lies within the boundaries of the chosen FOV.
Because CBCT's radiation dose is typically higher than that of conventional radiography, it is important to consider its diagnostic efficacy for each procedure to reduce radiation exposure. Radiation exposure to the patient may be reduced by reducing the size of the FOV. However, in real practices, it is much easier to acquire high quality images for a particular Region of Interest (ROI) using a larger FOV, which clearly results in increased radiation exposure. The difficulty associated with imaging a small ROI within a small FOV may be overcome if it is possible to easily center the FOV over the central axis of the ROI and thus reducing these doses by judicious adjustment of exposure factors and limiting the field of view to the smallest dimensions consistent with the clinical situation.
In recent years, many researchers have tried to expand the CBCT's use into facial and cranial reconstruction surgeries. Traditionally, reconstruction of head and facial injuries, are based only on post-traumatic 3-dimensional CT scans captured after the traumatic event, necessitates multiple operating room visits. These are required due to non-existent pre-injury images which would greatly assist in the fabrication of craniofacial implants to replace the missing structures. As a result implants must be custom fabricated, fitted and adjusted which is both time consuming and labor intensive. Additionally, due to the absence of any pre-injury information, the prolongation of treatment can lead to a less than optimum esthetic outcome. CBCT's ability to generate both hard and soft tissue images using a fraction of the radiation at a lower cost place it at an advantage over conventional CT scans and other three dimensional photogrammetric systems that capture the entire head.
These attributes also make CBCT ideal for use in military populations to establish pre-existing images of these tissues for deploying personnel. In the event of a traumatic injury, the patient's digital data can be retrieved. When used in conjunction with post-traumatic injury images, the information will permit the reconstructive team to formulate a complex treatment plan, fabricate custom guides and custom craniofacial implants prior to the first surgery.
However, CBCT also has several drawbacks which may adversely affect its efficiency and efficacy for craniofacial applications. Having accurate and reliable measurements of hard and soft tissues are especially important in facial/cranial reconstruction, such as the tissue thickness for specific landmarks of the face and scalp. CBCT acquisition typically requires about 60 seconds with a full gantry rotation, which covers about 15 breathing cycles. Thus, single breath-hold CBCT is impractical. Therefore, when organs move during respiration, CBCT can only generate a blurred composite organ volume rather than a true organ volume. Although some 4-D CBCT image acquisition techniques are being investigated, they either require long imaging time (such as segmented breath-hold acquisitions), deliver poor image quality (e.g., re-binning of projection data from a gated image acquisition), or excessive dose (e.g., respiratory re-sorting technique).
Another drawback of CBCT is the requirement of full rotational clearance of the gantry with respect to the position of the patient and couch. This is potentially problematic for alignment devices requiring substantial immobilization and patient support. Although smaller rotation angles (180 degrees plus a fan angle) may be reasonable for CBCT reconstruction, it still requires clearance of 360 degrees for gantry rotation. Yet another drawback of CBCT is its relatively high radiation dose, which can range from 2 to 9 cGy for optimal image quality. Repeat imaging will result in higher cumulative doses.
These limitations of CBCT are not due to CBCT itself, but are principally due to the mechanical limitations of patient alignment and localization. Thus there is a great need for a better patient positioning device, which allows easy alignment and localization of the patient while improving image quality and reducing patient's radiation exposure.
Existing and previous methods to position a human subject's head in a CBCT device include chin cups, oral bite tabs, head straps, and face nets (see U.S. Pat. No. 5,947,981, WO 2010089639, EP2278923 and US Pub No. 20010228907). These head positioning devices are normally supplied by the manufacturer of the CBCT device, and are generally specific to the device. These existing devices typically do not allow for fine adjustment of head position of the subject. They also fail to help the subject maintain a constant facial expression during and immediately after the CBCT scanning session, which could reduce image artifacts.
An example of the existing head positioning device are the three positioning attachments provided with the Kodak K9000/9300/9500 CBCT for use with the built in temple positioning guides, which include: a chin cup, an elastic head strap and a universal bite tab. The chin cup helps to quickly positions the subject. However, it impinges on, and deforms the chin. The elastic head strap helps to attach the device on patient's head but is incapable to fix the position of jaw or the gap between the teeth. The universal bite tab does not effectively resist movements of the head nor allow fine positing adjustment. As a result the CBCT scan images are often degraded by motion artifacts because of minor head moves. Important anatomy can be missed if positioning is not maintained. Furthermore, none of these existing devices fix nor maintain the facial expression of the subject.
This invention provides a head positioning device for properly aligning and localizing a patient's head and facial expression for procedures using a diagnostic imaging system, such as a cone-beam computed tomography (CBCT), or a single photon emission computed tomography (SPECT). However, it will be appreciated that the described technique may also find application in other imaging systems, other medical scenarios, or other medical techniques.